The present invention relates to a scavenging system for a two-stroke-cycle engine with a scavenge pump.
As the simplest method of producing scavenge air, a crankcase scavenging system is generally used.
In an ordinary crank-case-scavenged two-stroke engine, a vacuum is produced in an enclosed crankcase of the engine to draw in a fresh air when a piston ascends. When the piston descends, burned gas is discharged through an exhaust port while the fresh air in the crankcase is admitted into a cylinder, thereby scavenging the cylinder. However, at light engine load, since intake air quantity is small, the scavending becomes insufficient, resulting in misfire and hence in an irregular engine operation. In addition, a torque characteristic corresponding to the engine operating conditions becomes unstable. Moreover, at a heavy engine load, volume of the crankcase is insufficient for scavenging.
In order to resolve such problem, a two-stroke engine with a separate scavenge pump is used. Japanese Patent Application Laid-Open 57-203821 discloses a two-stroke engine which is provided with a scavenge pump. The scavenge pump is driven by the crankshaft to generate a scavenge pressure thereby delivering a sufficient amount of fresh air into the cylinder.
The engine is provided with a direct fuel injection system so as to prevent the fuel from escaping through the exhaust port.
Since the scavenge pump which is also used for reducing inertia of the reciprocating piston of the engine is driven at the same speed as the crankshaft, scavenge pressure becomes insufficient for scavenging in low speed range of the engine.